1. Field of the Invention
This invention relates to polymer topcoated, sulfur coated fertilizer granules or prills which are abrasion resistant, free flowing, non-smearing, essentially dustless fertilizer compositions having enhanced slow release characteristics.
2. Description of the Prior Art
The process for sulfur coating of fertilizer granules or prills was developed in 1968 by the Tennessee Valley Authority (TVA) in Muscle Shoals, Ala., as an economical system for reducing the rate of dissolution of urea particles when they are applied to the soil as fertilizer. U.S. Pat. No. 3,342,577 describes this sulfur coating process and also the sealant material that was necessary to fill in holes, cracks or imperfections that result naturally in the sulfur coating as it cools. The TVA process is also described in Sulphur Inst. J. 4(3) 2-6 (1968), ibid. 8(4) 2-5 (1972a) and ibid. 8(4) 6-8 (1972b). The TVA recommendation for the sealant is a mixture of 30% polyethylene resin in 70% bright stock mineral oil.
The process is further described in Sulfur Coating of Urea Treated with Atapulgite Clay, Gullett, G. L.; Simmons, C. L.; and Lee, R. G.; presented at the 198th American Chemical Society meeting in Miami Beach, Fla., in September 1989.
The requirement for a sealant for sulfur coated urea (SCU) has been documented by McClellan and Scheib (Sulphur Inst. J. 9(3/4) 8-12 (1973)), and by Scheib and McClellan ibid. 12(1) 2-5 (1976).
A description of slow release urea and NPK fertilizers is given in Hort. Rev. 1 79-140 (1979).
Paraffin waxes have been used to produce relatively slow dissolving clathrate complexes with urea by processes which do not relate to coating urea or other fertilizer granules or prills, but instead involve a solution or dispersion of urea in paraffin, as described in U.S. Pat. No. 3,252,786.
Paraffin alone has not been used as a slow release coating for fertilizers such as urea because of its lack of adhesion. Also, paraffin has not been used as a coating for sulfur coated fertilizers such as SCU for the same reason and, also, because the coating is rubbed off or otherwise cracked or abraded when the sulfur coated fertilizer prills or granules are handled in high speed bulk moving equipment wherein large amounts of product are moved by hopper car or by truck and off-loaded into storage bins. The wax-oil sealants as described by TVA publications and currently being applied to SCU require the addition of a clay conditioning agent at levels nearly equal to that of the wax sealant to prevent caking and provide a free flowing product. A typical process may require 3% of the wax-oil sealant and 2% of the clay conditioning agent on the weight of the SCU, or 67% clay on the weight of the sealant. This requires large scale clay handling equipment in addition to the equipment necessary to apply the molten sulfur and wax sealant.
When the clay conditioned SCU is applied in the field by mechanical spreaders, the clay or wax-clay mixtures tend to be removed from the SCU particles by the abrasive action of the screw conveyors and the mechanical spreading wheel, resulting in a build-up of wax-clay on various parts of the machine and requiring frequent shut-downs for cleaning.
An additional problem with SCU from current production methods is the reduction or loss in WIN which occurs on shipping the SCU from the production point to the local blending point. This loss is exacerbated by the further abrasion which occurs in the blending and bagging operations. When they are mixed and bagged, urea and blended fertilizers containing slow or controlled release nutrients must be labelled with their WIN content, and it is the responsibility of the manufacturer to assure that the value does not decrease below the labeled value during shipping and storage. Thus a significant loss in WIN resulting from handling in modern high-speed equipment can create a situation in which the fertilizer is mislabeled and subject to recall.
U.S. Pat. No. 3,372,019 describes wax/resin coated fertilizer compositions wherein the wax/resin coating composition is applied directly onto the surface of a fertilizer substrate and not onto sulfur coated fertilizer substrates.
U.S. Pat. No. 4,881,963 describes polymer coated fertilizers wherein the polymer coating includes an ethylene--carbon monoxide copolymer as an indispensable component. The coating may also contain another resin including rubbery resins such as natural rubber, polyisoprene, polybutadiene and the like or an ethylene--vinyl acetate copolymer. However, again, the polymer coatings are not applied over primary sulfur coated fertilizers.
U.S. Pat. Nos. 4,042,366 and 4,676,821 teach the use of standard TVA-type coatings applied over sulfur coated particulate cores. In both of these patents, the topcoat is relatively soft and flowable at ambient temperatures so that the material acts as a sealant to fill voids, cracks or imperfections inherent in the surface of the sulfur layer. These patents do not describe the use of non-flowable topcoats over the sulfur coating. Furthermore, these compositions require the application of dust-like conditioning agents in or over the sealant layer.
U.S. Pat. No. 3,576,613 describes sulfur coated fertilizer product consisting of: (a) an inner core comprising a solid fertilizer pellet; (b) a subcoating layer immediately adjacent to and surrounding the inner core fertilizer pellet; and (c) a coating of elemental sulfur substantially completely encapsulating the subcoated fertilizer pellet. The subcoating layer interposed between the fertilizer core and the sulfur coating is required to achieve the desired results and comprises a finely divided powder such as charcoal or carbon black capable of reducing the contact angle between the surfaces of the underlying fertilizer pellet and the outer sulfur coating.
U.S. Pat. No. 3,903,333 describes a method for producing slow-release fertilizers coated solely with sulfur and the intent of the invention is to eliminate the need for a secondary coating or sealant application over the primary sulfur coat.